Interactive tone mapping for high dynamic range video

ABSTRACT

A user interactive video tone mapping method is provided that can be used for the tone reproduction of high dynamic range (HDR) content on low dynamic range (LDR) displays. The method comprises the steps of selecting a frame of a video sequence; providing a user interface for a user to manually apply scribble marks to the frame; manually applying N scribble marks to the frame, N being a whole number of value 1 or in greater; and tone mapping the video sequence responsive to the scribble marks.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application Ser.No. 61/276,626, filed Sep. 14, 2009, which is incorporated by referenceherein in its entirety.

FIELD OF THE INVENTION

The invention relates to the tone reproduction of high dynamic range(HDR) content on low dynamic range (LDR) displays and provides animproved method of tone reproduction driven by user scribe marks toselected frames of video.

BACKGROUND OF THE INVENTION

High dynamic range (HDR) displays are displays that can display imagerywith very high contrast, very deep blacks and very bright whites. Suchtypes of displays can show HDR imagery by using non-uniformbacklighting. In particular, one can adjust the intensity of thebacklighting on different areas of the screen based on the input image.These displays have received much attention in the recent years as analternative format for digital imaging.

The traditional Low Dynamic Range (LDR) image format was designed fordisplays compliant with ITU-R Recommendation BT 709 (a.k.a. Rec. 709),where only two orders of magnitude of dynamic range can be achieved.However, real world scenes have a much higher dynamic range which arearound ten orders of magnitude in daytime. The human visual system (HVS)is capable of perceiving 5 orders of magnitude.

Because most display devices have a limited dynamic range, HDR scenesshown on Low Dynamic Range (LDR) display devices usually turn out to beeither saturated (corresponding to the concept of “overexposure” inphotography) or extremely dark (corresponding to “underexposure”).Either case is undesired as numerous details can be lost. Therefore, aprocess called tone mapping or tone reproduction is needed to let theHDR contents be displayable on traditional display devices.

Tone mapping for HDR video has drawn much attention in academia as wellas in industry. However, compared with the tone mapping of still images,relatively very little effort has been put on HDR video tone mapping.Particularly, tone mapping for HDR images has been studied in recentyears in computer graphics as well as in image and video processing.Roughly speaking, tone mapping methods can be classified into twoprimary categories: global tone mapping and local tone mapping.

Although local tone mapping using a gradient domain method to video hasadded a smoothness constraint term along the motion direction whichhelps to reduce temporal brightness fluctuation, the control of theimage appearance is limited as this mapping method tends to generateimages with artificial look and seems to have a limited number of waysto control the image appearance.

One publication (Interactive Local Adjustment of Tonal Values by DaniLischinski, Zeev Farbman, Matt Uyttendaele, Richard Szeliski in ACMTransactions on Graphics, 25(3) (Proc. ACM SIGGRAPH 2006), July 2006)provided a scribble based interactive tone mapping approach for HDRimages. This method shows flexibility in terms of changing theappearance of image. However, the extension of such a method to HDRvideo is very difficult because it is not practical to draw scribblemarks on each frame. Tracking scribbles along temporal axis is also notfeasible if there exists complex motion.

As such, a need exists to develop a method for HDR video tone mappingthat incorporates user interactivity to improve the tone mappingquality, but yet does not require the user to directly mark each framein a video sequence to achieve high quality tone mapping.

SUMMARY OF THE INVENTION

A user interactive video tone mapping method is provided that can beused for the tone reproduction of high dynamic range (HDR) content onlow dynamic range (LDR) displays. The method comprises the steps ofselecting a frame of a video sequence; providing a user interface for auser to manually apply scribble marks to the frame; manually applying Nscribble marks to the frame, N being a whole number of value 1 orgreater; and tone mapping the video sequence responsive to the scribblemarks. The method can further include classifying pixels in the frame toN categories in which the N categories correlate to the N scribble marksand the classifying can be applied to all frames in the video sequence.The method can further include generating an initial exposure map forthe frame, wherein the initial exposure map is used to classify thepixel in the classifying step; applying an edge preserving filter togenerate a final exposure map, wherein the tone mapping step is furtherperformed responsive to the final exposure map; add creating featurevectors from initial input pixel data of the video sequence for eachpixel along the N scribe marks, such that the classifying step isresponsive to the feature vectors. The method further can include usingone Gaussian mixture model in creating the feature vectors, wherein thefeature vectors of the pixels at least include three luminance values, afirst value for the luminance of a given pixel itself, and the othersbeing average luminance of the neighboring pixels using different windowsizes.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described by way of example with reference tothe accompanying figures of which:

FIG. 1 is a flow diagram of the user interactive video tone mappingmethod according to the invention; and

FIG. 2 is a video frame showing the user scribble marks according to theinvention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The invention which will now be described is essentially a userinteractive video tone mapping method that can be used for the tonereproduction of high dynamic range (HDR) content on low dynamic range(LDR) displays. A key feature of the invention is a user interfaceadapted to permit the user to access a sequence of frames of video data,select or determine one key frame of the sequence, and apply or draw oneor more scribbles or marks to the key frame in order enhance or modifyfeatures in specific locations of a frame responsive ideally to thepreference and/or desire of the user. This user input information isthen propagated across the output video sequence automatically, whichgreatly reduces the manual input load. Meanwhile, the flexibility ofchanging the photographic appearance is still preserved.

Based on the input scribbles in the keyframe, the method can trainseveral Gaussian Mixture Models (GMM) to capture the information of thescribbles. Afterwards, all the pixels in the video are classified usingthe trained GMM models and the initial exposure values are set for allpixels. An edge preserving filtering can then be applied to smooth theexposure map of each frame to obtain the final exposure map for eachframe. The exposure map can be a floating point matrix that contains theexposure value for each pixel in the HDR image. The tone mapped HDRvideo is then obtained by dividing each HDR frame by its correspondingexposure map.

The method can be characterized as a stable and flexible interactivevideo tone mapping algorithm, wherein the method ultimately estimates anexposure map for each frame. The exposure map is a two dimension matrixthat has the same size as the picture. Each entry of the exposure mapcan be seen as the exposure value for the pixel at that location. Thetone mapped video can be obtained by dividing the original video frameby the corresponding exposure map.

For the exposure map estimation, the method provides a means wherein theapplication is not required use the user-defined scribbles as directcontraints. Instead, the application can consider the scribbles whichhave been assigned with different exposure values as indicators fordifferent categories (for instance, N scribbles correspond to Ncategories). FIG. 2 gives an example of HDR image marked with threescribbles (which can be non-continuous) which are shown in threedifferent reference; however, in the actual software the three differentscribbles are identified by different colors. (The scribbles can also becontinuous and can be close-looped or open-looped.) The idea here is toclassify all pixels in one frame into these N categories. Once theclassification is done, each pixel can be associated with an estimatedexposure value, which will be served as initial guess of the desiredexposure map. The exposure map of each frame can be obtained by an edgepreserving filtering on the previously obtained initial guess ofexposure map.

To classify the pixels into N categories, the user can first form afeature vector for each pixel on the scribbles. These feature vectorscan be obtained in a number of ways such as local averaging in differentscales. The GMM models can be trained from these feature vectors.Finally the GMM model can be used as a classifier to classify all pixelsin one frame into N categories.

The proposed method is a first attempt for interactive tone mapping forHDR video. It has advantages such as reasonable user input load as wellas the flexibility to change the video appearance. In comparison,trivial extension of the interactive tone mapping can involve drawingscribbles on each frame, which needs tremendous user input load. Themethod can also avoid the tracking scribbles over time, which can be thesource of temporal inconsistency if not well treated.

A preferred embodiment of the invention will now be described withreference to FIG. 1. The embodiment is essentially a video tone mappingworkflow that can begin with the step of accessing HDR video 105 andthen selecting a key frame 110 from a sequence of frames making up ascene. Here, through an appropriate user interface, the user can togglethrough frames and select a key frame to which the user will drawscribbles/strokes/marks. Optionally, the key frame can also berecommended by a system automatically through some alogorithm or thelike. This could be the first frame, the middle frame or a frame meetingsome predetermined statistical requirement such as being the frame withthe closest to average luminanse, mean luminance, etc.

Next, the user can generate or make scribbles 115 by using the interface(which can be a computer with appropriate display or the like and amouse or track ball or the like for the actual drawing of scribbles on adisplayed frame). The user can draw N scribbles on the key frame andthen assign exposure values to each respective scribble. These exposurevalues will be used in step 135. The interface can have an appropriatealogorithm which asks the user if more scribbles are desired and asksthe user automatically to assign exposure values.

An example of the user-defined scribble is shown as in FIG. 2, whichshows a key frame 200. In FIG. 2, there are numermous scribbles made bythe user; however, in the current example there are actually only threesets of scribble 205, 210, 215. The first set 205 are scribble marksmade by the user in which those marks will be treated the same, thesecond set 210 of scribble marks made by the user in which those markswill be treated the same, and likewise the third set 215 of scribblemarks made by the user in which those marks will be treated the same.The different sets of scribble can be treated differently from oneanother. In operation with the user interface, the different sets ofscribble marks can have different colors to remind the user what marksare associated with what set. Note that in the figure there are otherscribble marks made by the user which are not labeled to keep theexample simple.

The next step can be the creation of feature vectors 120 for the pixelsalong each scribble. These vectors can be computed or determined fromthe actual input signal for the frame. For every pixel in each category,a feature vector needs to be computed for GMM training. Similar to otherrecognition or classification applications, the feature vector should bedesigned in such a way that it can capture the unique characteristics ofeach category. The simplest form of feature vector is to use theluminance value of the pixel. In this case, feature vector is aone-by-one vector. RGB color coordinate of one pixel can also be used asa 3-by-1 feature vector. More complicated feature vectors canincorporate spatial information as well. Another embodiment involves thecomputation of the feature vector as multi-scale average around thespecific pixel. For instance, the user can construct a feature vector as(L1, L2, L3) for a pixel where L1 is the luminance value of the pixelitself, L2 is the average luminance of all pixels inside a 5-by-5 windowcentered with this pixel and L3 is the average luminance of all pixelsinside a 9-by-9 window centered with the pixel. This feature vector canin some sense capture information of a given pixel at different scales.This choice is made in favor of speed. A Gaussian filtering or othernonlinear filtering can be used instead more costly and complex filters.

Following the creation of feature vectors is the step of GaussianMixture Model (GMM) training 125 of N (number of scribbles) GMM models.Here models are trained using corresponding feature vectors. In theexample in FIG. 2. N is 3. For each category, one GMM model is trainedbased on the feature vectors of pixels that belong to the same category.Take the first set of scribbles 205 in FIG. (which could be the shown asbeing in red). First, the feature vectors for all the pixels along thefirst set are computed as described in Step 115. Then, the user caninitialize the GMM model training by specifying the number of Gaussiansin the GMM model. Afterwards a standard GMM training algorithm can beused to estimate the parameters of the GMM model.

GMM training can be done with any off-the-shelf GMM parameter estimationmethod, such as Expectation Maximization algorithm disclosed byChristopher M. Bishop in the textbook “Pattern Recognition and MachineLearning,” (Springer, October 2007). The same process is repeated forevery category to train a separate GMM model. For example, as shown inFIG. 2, three GMM models are obtained for the first set 205, the secondset 210, and the third set 215 of scribbles, respectively.

Following the training step is the step of pixel classification 130 inwhich all the pixels are classified into N categories by using thetrained GMM models. Once the GMM models are obtained from training, theuser can apply these models to classify all the pixels into Ncategories. The classification is done by computing the likelihood ofeach pixel belonging to each category. This can be done by making ahistogram of the pixels according to luminance and assigning probabilityparameters or weights to the individual pixels so that they can be incertain luminance regions. The pixel will be put into the category withthe largest likelihood.

Next is the step of exposure map computation 135. Here, the initialguess of the exposure map is obtained by setting the same exposure value(assigned to each scribble from the user input) for all pixels in thesame category. The final exposure map is computed by applying an edgepreserving filtering on this initial guess. This step computes theexposure map for each frame. In previous steps, all pixels areclassified into N categories and a user-defined exposure value isdesignated for each category. Hence, the user can assign the sameuser-defined exposure value to all pixels in the same category, toobtain an initial guess for the exposure map.

Afterwards, a smooth filtering can be applied based on the initial guessof exposure map. An edge preserving nonlinear filter could be used forthis purpose. For instance, bilateral filtering or weighted least squarefiltering can be good candidates for smooth filtering. The weightedleast square filter is known for halo artifact avoidance and is moresuitable for this step. More specifically, the user can smooth theinitial guess of exposure map by optimizing the function:

$J = {\left( {f - g} \right)^{2} + {\lambda\left( {\frac{\left( \frac{\partial f}{\partial x} \right)^{2}}{{L_{x}}^{\beta} + ɛ} + \frac{\left( \frac{\partial f}{\partial y} \right)^{2}}{{L_{y}}^{\beta} + ɛ}} \right)}}$

where f is the exposure map to be estimated, g is the initial guess ofexposure map. L_(x) and L_(y) are first order derivative of the HDRimage in horizontal and vertical direction respectively. β and ε are twoconstants, typically taking values of 1.8 and 1e-5 respectively. λ is aLagrange multiplier, which is selected to balance the first term and thesecond term. Many well studied algorithms such as conjugate gradientdescent can be used to accelerate the computation of finding theapproximate solution of the above optimization problem.

Next is the step of tone mapping 140. Here tone mapping of the video iscomputated by dividing the original HDR frames by corresponding exposuremap computed in step 135. The tone mapped frame can further be processedwith a gamma correction step. This can be described as:

I=(H/E)^(1/γ)

where H is the HDR frame, E is the exposure map and γ is a constant,typically takes values in the range [2.2,2.4] and represents the gammaof the output device (where the tone mapped video will be shown).

The method can further include a quality control check 145 in which theuser can look at the tone mapped frames and, if satisfied, accept theselected set of parameters and complete the tone mapping 160. If theuser is not satisfied, the user can provide more input adjustments. Inother words, optionally, the user can check the look of the tone mappedvideo and make changes to the exposure value for the scribbles 155 andthen repeat step 135 until the result is satisfactory. If the user wouldlike to add another scribble in step 150, this is also possible, whichis effectively repeating steps 115 to 140 until a satisfactory result isobtained.

The foregoing illustrates some of the possibilities for practicing theinvention. Many other embodiments are possible within the scope andspirit of the invention. It is, therefore, intended that the foregoingdescription be regarded as illustrative rather than limiting, and thatthe scope of the invention is given by the appended claims together withtheir full range of equivalents.

1. A method comprising the steps of: selecting a frame of a videosequence; providing a user interface for a user to manually applyscribble marks to the frame; manually applying N scribble marks to theframe, N being a whole number of value 1 or greater; and tone mappingthe video sequence responsive to the scribble marks.
 2. The method ofclaim 1 comprising: classifying pixels in the frame to N categories, theN categories correlate to the N scribble marks.
 3. The method of claim 1comprising: classifying pixels in the frame to N categories, the Ncategories correlate to the N scribble marks, wherein the classifying isapplied to all frames in the video sequence.
 4. The method of claim 3comprising: generating an initial exposure map for the frame, whereinthe initial exposure map is used to classify the pixel in theclassifying step.
 5. The method of claim 4 comprising: applying an edgepreserving filter to generate a final exposure map, wherein the tonemapping step is further performed responsive to the final exposure map.6. The method of claim 3 comprising: creating feature vectors frominitial input pixel data of the video sequence for each pixel along theN scribe marks, wherein classifying step is responsive to the featurevectors.
 7. The method of claim 6 comprising: applying at least oneGaussian mixture model in creating the feature vectors.
 8. The method ofclaim 6 comprising: applying at least one Gaussian mixture model increating the feature vectors, wherein the feature vectors of the pixelsat least include three luminance values, a first value for the luminanceof a given pixel itself, and the others being average luminance of theneighboring pixels using different window sizes.
 9. The method of claim6 comprising: applying an edge preserving filter to generate a finalexposure map for each frame, wherein the tone mapping is performed byapplying the final exposure map for each frame.
 10. The method of claim4 comprising: applying an edge preserving filter to generate a finalexposure map, wherein the edge preserving filter is a bilateral filteror a weight least square filter; wherein the tone mapping is performedby applying the final exposure map for each frame.
 11. The method ofclaim 1, wherein the N scribble marks is non-continuous.